In many rotary machine applications, it is necessary to detect rotor position. For example, where a brushless DC motor is to be operated, motor shaft position must be detected so that a power semiconductor switching network in the form of an inverter can be properly controlled to commutate the currents in the motor windings. In most simple motor controls, position information is only required every 60 electrical degrees. Many commercial and industrial brushless DC drives employ signals from Hall-effect devices, which detect the magnetic field of the motor permanent magnets. There are applications, however, where Hall-effect devices cannot meet performance requirements, for example, in the aerospace industry where severe environments are encountered. In these applications, synchros are used to sense motor shaft position. These devices are basically rotary transformers with three-phase sine wave voltage outputs having a trigonometric relationship to motor position and which provide infinite resolution of shaft angle.
It would be desirable to use the rugged synchro position sensor with a low cost six step motor drive. In this case, a logic conversion circuit must be provided to adapt the output of the synchro to the motor drive. However, the only devices currently available to perform this function are cost prohibitive, since they cost as much or more than synchro-to-digital converters, which are typically used in applications requiring a high degree of controllability beyond that required in the aerospace industry. Present conversion devices also exhibit significant power dissipation, primarily from the circuits used to excite the primary winding of this synchro. Also, present devices develop false logic transitions and error, particularly at low and high motor speed extremes. These problems can cause errant communication and/or higher switching losses in the inverter.
The need exists for a low cost synchro-to-logic converter which does not require pure sine wave excitation, and which exhibits positive logic transitions and accurate position sensing over a wide range of motor speeds.